CHAPTER VI. 



THE ORDOVICIAN (LOWER SILURIAN) PERIOD. 



Formations and Physical History. 



That no considerable physical change took place in the relations 

 of land and water in North America at the close of the Cambrian period 

 as usually defined is shown by the general conformity between the 

 Ordovician and Cambrian systems. Where unconformity exists, it is 

 relatively slight. There is here no considerable "lost interval " during 

 which the record of sedimentation is unknown, and the map which 

 shows the relations of land and water in our continent at the close 

 of the Cambrian (Fig. 95) serves also to indicate their relations at 

 the opening of the Ordovician. 



It has been seen that during the Cambrian period, so far as North 

 America is concerned, the sea slowly encroached on the land. During 

 the Ordovician period which followed, the climax of the transgression 

 was reached, and an epicontinental sea (Vol. I, p. 11) stood over much 

 of the continent (Fig. 129). This general statement is not to be con- 

 strued to mean that the continental area was altogether without move- 

 ment during this period. There is reason to believe that temporary 

 and local oscillations of level, either of the land or of the sea, or of 

 both, sufficient to change conditions of sedimentation, were repeated 

 many times and in many places during the period, and that at its 

 close, geographic changes of great importance took place. 



Sedimentation During the Ordovician Period. 



So long as the sea was encroaching on the land, as in the Cam- 

 brian period, the sediments deposited within the continen al area 

 were predominantly clastic. Deposits of organic sediments (shells, 

 coral, etc., and their comminuted products), together with more or 

 less land-derived detritus, were being made in the clearer waters beyond 



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